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When setting several variables in a row, be aware that the order of the
evaluation is undefined. For instance ‘foo=1 foo=2; echo $foo’
gives ‘1’ with Solaris 10 /bin/sh
, but ‘2’ with Bash.
You must use
‘;’ to enforce the order: ‘foo=1; foo=2; echo $foo’.
Don’t rely on the following to find subdir/program:
PATH=subdir$PATH_SEPARATOR$PATH program
as this does not work with Zsh 3.0.6. Use something like this instead:
(PATH=subdir$PATH_SEPARATOR$PATH; export PATH; exec program)
Don’t rely on the exit status of an assignment: Ash 0.2 does not change the status and propagates that of the last statement:
$ false || foo=bar; echo $? 1 $ false || foo=`:`; echo $? 0
and to make things even worse, QNX 4.25 just sets the exit status to 0 in any case:
$ foo=`exit 1`; echo $? 0
To assign default values, follow this algorithm:
: "${var='my literal'}"
: ${var="$default"}
var=${var="$default"}
test ${var+y} || var="has a '}'"
In most cases ‘var=${var="$default"}’ is fine, but in case of doubt, just use the last form. See Shell Substitutions, items ‘${var:-value}’ and ‘${var=value}’ for the rationale.
Next: Parentheses in Shell Scripts, Previous: Shell Substitutions, Up: Portable Shell Programming [Contents][Index]